What is Diorite?

Diorite is an intrusive igneous rock and is coarse grained because of the greater time for settling. It is between granite and gabbro and is similar to the fine grained extrusive rock, andesite. Diorite is composed mainly of plagioclase feldspar with amphibole and pyroxene minerals. The black minerals in diorite are hornblende and the whote mineral is plagioclase feldspar. The diorite has no or very little amount of quartz unlike granite. The sodic plagioclase minerals of the diorite gives it a high relief look.

Diorite is the name used for a group of coarse-grained igneous rocks with a composition between that of granite and basalt. It usually occurs as large intrusions, dikes, and sills within continental crust. These often form above a convergent plate boundary where an oceanic plate subducts beneath a continental plate.

Partial melting of the oceanic plate produces a basaltic magma that rises and intrudes the granitic rock of the continental plate. There, the basaltic magma mixes with granitic magmas or melts granitic rock as it ascends through the continental plate. This produces a melt that is intermediate in composition between basalt and granite. Diorite forms if this type of melt crystallises below the surface.

Diorite is usually composed of sodium-rich plagioclase with lesser amounts of hornblende and biotite. It usually contains little if any quartz. This makes diorite a coarse-grained rock with a contrasting mix of black and white mineral grains. Students often use this "salt and pepper" appearance as a clue to the identification of diorite.

Diorite and Andesite

Diorite and andesite are similar rocks. They have the same mineral composition and occur in the same geographic areas. The differences are in their grain sizes and their rates of cooling. Diorite crystallised slowly within the Earth. That slow cooling produced a coarse grain size. Andesite forms when a similar magma crystallises quickly at Earth's surface. That rapid cooling produces a rock with small crystals.

This chart illustrates the generalized mineral composition of igneous rocks. It shows that diorites and andesites are composed mainly of plagioclase feldspar, amphiboles, and micas; sometimes with minor amounts of orthoclase, quartz, or pyroxene.

Diorite classification

The classification of diorite can be by the minerals constituent of the rock as, with a little quartz it becomes quartz diorite and with more quartz it become tonalite. With more alkali feldspar it becomes monzonite and when both minerals are abundant then the diorite becomes granodiorite.

Diorite appearance

Diorite has white and black minerals which gives it salt and pepper appearance. The identification of diorite in the field can be by using a hand lens to look for plagioclase minerals intersecting at oblique angles.

Occurrence

Diorite is a relatively rare rock; source localities include Leicestershire (one name for microdiorite-markfieldite-exists due to the rock's being found in the village of Markfield) and Aberdeenshire, UK; Guernsey; Sondrio, Italy; Thuringia and Saxony in Germany; Finland; Romania; Northeastern Turkey; central Sweden; the Darrans range of New Zealand; the Andes Mountains. An orbicular variety found in Corsica is called corsite.

Uses of Diorite

In areas where diorite occurs near the surface, it is sometimes mined for use as a crushed stone. It has a durability that compares favourably to granite and trap rock. It is used as a base material in the construction of roads, buildings, and parking areas. It is also used as a drainage stone and for erosion control.

In the dimension stone industry, diorite is often cut into facing stone, tile, ashlars, blocking, pavers, curbing, and a variety of dimension stone products. These are used as construction stone, or polished and used as architectural stone. Diorite was used as a structural stone by the Inca and Mayan civilisations of South America and by many ancient civilisations in the Middle East.

In the dimension stone industry, diorite is sold as a "granite." The dimension stone industry uses the name "granite" for any rock with visible, interlocking grains of feldspar. This simplifies discussions with customers who do not know how to identify igneous and metamorphic rocks.

Historic use

Diorite is an extremely hard rock, making it difficult to carve grand work with. It is so hard that ancient civilisations (such as Ancient Egypt) used diorite balls to work granite. Its hardness, however, also allows it to be worked finely and take a high polish, and to provide a durable finished work.

One comparatively frequent use of diorite was for inscription, as it is easier to carve in relief than in three-dimensional statuary. Perhaps the most famous diorite work extant is the Code of Hammurabi, inscribed upon a 2.23 m (7 ft 4 in) pillar of black diorite. The original can be seen today in Paris' Musée du Louvre. The use of diorite in art was most important among very early Middle Eastern civilisations such as Ancient Egypt, Babylonia, Assyria, and Sumer. It was so valued in early times that the first great Mesopotamian empire, the Empire of Sargon of Akkad, listed the taking of diorite as a purpose of military expeditions.

Although one can find diorite art from later periods, it became more popular as a structural stone and was frequently used as pavement due to its durability. Diorite was used by both the Inca and Mayan civilisations, but mostly for fortress walls, weaponry, etc. It was especially popular with medieval Islamic builders. In later times, diorite was commonly used as cobblestone; today many diorite cobblestone streets can be found in England, Guernsey and Scotland, and scattered throughout the world in such places as Ecuador and China. Although diorite is rough-textured in nature, its ability to take a polish can be seen in the diorite steps of St. Paul's Cathedral, London, where centuries of foot traffic have polished the steps to a sheen.